Radiation detectors for detecting high energy photons (e.g., gamma rays and x-rays)are well-known in the art and are used to detect high energy photons produced by any of a wide range of radioactive or other types of samples. The detection, identification, and spectroscopy of such energetic photons comprises an integral part of the fields of nuclear and particle physics as well as several fields that make use of radioactivity, including, for example, medicine, forensic science, and industrial inspection applications. Radiation detectors are also used at nuclear power plants and laboratories to monitor and study radiation levels.
This invention is particularly suited for use with two types of ionizing radiation detectors. The first type, referred to herein as "gas tube" or simply "gas" detectors utilize a gas-filled chamber or tube which contains a positively charged wire. When a high energy photon enters the chamber it may ionize a gas atom, causing it to release an electron or electrons in the process. The liberated electron or electrons may in turn ionize additional gas atoms, which liberate yet more electrons. The liberated electrons are collected by the positively charged wire. A detection circuit connected to the wire measures the charge delivered to the wire by the electrons. Generally speaking, the higher the energy of the incoming photon, the more atoms are ionized and the more electrons are liberated. Therefore, the magnitude of the detected charge is related to the amount of energy lost by the incoming photon inside the detector. If the photon loses all of its energy in the detector, the magnitude of the detected charge is proportional to the photon energy.
Solid state detectors are similar to gas detectors except that the active volume (i.e., the gas) is replaced by a semiconducting material such as germanium. Accordingly, both types of detectors have in common the property that they use the energy of the incoming photon to ionize an atom of some material. Generally speaking, solid state detectors provide superior sensitivity and resolution compared with gas tube detectors, although both types remain in use. A major difference between gas and solid state detectors is that gas detectors generally multiply the liberated charge, while solid state detectors generally do not.
Regardless of the type of radiation detector that is used in a given application, spectrometric measurements (i.e., the measurement of the energy distribution of the incoming photons) may be complicated by factors such as "pile-up" and "dead time." Pile-up may occur when two separate photons enter the detector at approximately the same instant, in which case the total charge (i.e., ionization or pair production) may be greater than the charge that would be produced by either of the photons alone. Dead time refers to those periods in which a signal processing system (which may include an analog-to-digital converter) associated with the detector is processing a signal (e.g., a pulse) resulting from a photon. The signal processing system may be unable to accept or process additional signals or pulses produced by subsequent photons during this processing or "dead" time, resulting in a loss of such additional signals. Fortunately, however, systems and methods have been developed that compensate for factors such as pile-up and dead time.
One system and method that may be used to correct for pile-up and dead time utilizes a pulse generator circuit to inject a plurality of pulses into the test input of the detector pre-amplifier. Since the frequency and amplitude of the injected pulses are known, the signal processing system associated with the detector amplifier may identify certain of the data signals received from the detector amplifier as those produced by the pulse generator. Since the number and timing of the injected pulses from the pulse generator are known, the fraction of data lost due to pile-up and dead time can be calculated. A correction factor may then be used to correct for lost data. Also, since the pulses from the pulse generator are very stable in amplitude over time, they can be used to provide a calibrated energy scale for each acquired spectrum. (This is particularly applicable with solid state detectors.) Because the pulses produced by the pulse generator may be separated from detector pulses, they may be used to detect slight changes in the gain or zero of the energy scale before they are observed otherwise.
While such pulse injection systems are useful in improving the performance of radiation detectors, they are not without their problems and disadvantages. For example, one problem with the pulse injection method is that care must be exercised to ensure that the injected signals are positioned in an uncluttered or unused spectral region. This is especially difficult if the pulse generator provides pulses having both high and low amplitudes. This difficulty generally limits detectors having dual amplitude pulse injection systems to applications in which the spectrum of the source is known or has been previously determined.
A system and method which solves the foregoing difficulty is described in U.S. Pat. No. 4,968,889, entitled "Pulser Injection with Subsequent Removal for Gamma-Ray Spectroscopy," which is incorporated herein by reference for all that it discloses. Briefly, this patent discloses a pulser control and separation logic module which controls the injection of the pulses and includes separation logic which enables the injected pulses to be stored in a region of the multichannel analyzer that is separate from the region reserved for the storage of x-ray and gamma ray events. While the system and method described in the foregoing patent provides improved compensation for pile-up, dead-time, gain, and zero point shifts, it is not particularly well-suited for portable applications wherein the power available to operate the detector and pulse generator circuits is limited (e.g., batteries). Another problem associated with portable applications is that the detector and pulse generator circuits are often subjected to substantial temperature fluctuations and other environmental factors which can adversely affect the accuracy and stability of the injected pulses.
Consequently, a need remains for a pulse generator having reduced energy consumption while at the same time providing increased pulse accuracy and stability over a wide range of temperatures and environmental conditions.